The reasons construction documents must section through anything built into the interior is to explain not only the assembly but the level of workmanship expected.

If plywood construction with a birch veneer is adequate, the specifications and drawings must so indicate.

This seven dollar tool, a contour guide, is worth its weight in gold.   It allows you to reproduce, most precisely, a molding profile by adapting to its shape.  In historical preservation projects it is an indispensable instrument.